In general, semiconductor high-purity chemicals and ordinary chemicals are stored in a tank good in resistance to these chemicals. As shown in FIG. 11, hitherto it has been known in the art that: formed in this tank 70 are two pieces of mouth stopper portions 71 and 71a; and, plugs 72 and 72a are mounted on these mouth stopper portions 71 and 71a, respectively, wherein each of the plugs 72 and 72a is provided with a plug portion not shown in the drawings. These plugs 72 and 72a function as lids for closing the mouth stopper portions 71 and 71a, respectively. Of these plugs 72 and 72a: one serves as a liquid passage connecting piece 72; and, the other serves as a gas passage connecting piece 72a. Connected with the liquid passage connecting piece 72 is a siphon tube 73 which reaches in the vicinity of a bottom portion of the tank 70.
Further, in this kind of tank 70, a cap is mounted on a plug portion of each of the plugs 72 and 72a, which are mounted on the mouth stopper portions 71 and 71a of the tank, respectively, so that a through hole formed in each of the plug portions is closed by means of this cap. Under such circumstances, the tank is carried or reserved.
In retrieving the liquid stored in this tank 70, first of all, the caps having been mounted on the plug portions of the plugs 72, 72a provided in the mouth stopper portions 71, 71a in the tank 70 are removed. After that, each of sockets 75, 75a having been connected with a hose 74 is connected with each of the plug portions of these plugs 72, so that the liquid contained in the tank 70 is retrieved and supplied to the hose 74 through the siphon tube 73 by using a pump P. At this time, in order to prevent the interior of the tank 70 from falling in negative in pressure, an inert gas is supplied to the tank 70 from a hose 74a having been connected with the plug portion of the other plug 72a. 
In some instance, the liquid stored in the tank 70 vaporizes with the lapse of time. Consequently, there is a fear that the pressure inside the tank 70 increases in use. Due to this, a tank and a plug, which is used in the tank, each improved in safety and free from a fear that the liquid flows out of the tank under the influence of the pressure inside the tank 70 in use, have been proposed (Japanese Patent application Laid-Open No.: 2002-114242 gazette).
Further, in Japanese Patent application Laid-Open No.: 2002-54782 gazette, there is disclosed a tube joint which is capable of: reducing the amount of air mixing into the liquid when the socket is connected with the plug as small as possible; and, preventing the liquid from leaking from the socket and the plug when the socket is separated from the plug.
Incidentally, in order to prevent the sludge having been precipitated out of a liquid chemical in the tank from increasing in volume, in recent years, the need to circulate the liquid chemical at a time when the liquid chemical stored in the tank is not delivered exists in the art. In this circulation of the liquid chemical, the liquid chemical having been retrieved from the tank through the siphon tube is returned to the interior of the tank. In a conventional tank, its gas plug which is small in its inner diameter suffices only for pressure feed of the gas. However, such gas plug is too small in diameter when used also in circulation of the liquid chemical, and therefore can't be used to realize a smooth circulation of the liquid chemical.
Patent document 1: Japanese Patent application Laid-Open No.: 2002-114242 gazette; and
Patent document 2: Japanese Patent application Laid-Open No.: 2002-54782 gazette.